High styrene and high vinyl SSBR is difficult to produce due to the kinetics of copolymerization. Typically, polar agents known as randomizers are added to the polymerization system in order to achieve high vinyl content and random styrene incorporation.
Tetramethylethylenediamine (TMEDA) is one such randomizer. However, as demonstrated by Comparative Examples 1 and 2 described below, TMEDA results in a significantly broader molecular weight distribution than is typically observed for anionic polymerization reactions when utilized for the polymerization of high styrene (styrene ratio >40%) SSBR—especially in the polymerization of rubber with high molecular weights.
Broad molecular weight distributions in anionic batch polymerization can result from chain transfer, metallation, and/or slow initiation reactions. These reactions reduce the amount of polymer chains that are still living at the conclusion of the polymerization reaction. Nowadays, high-performance SSBR is typically chain-end modified. Although such chain-end modification can significantly improve overall performance of a compound, it requires that the amount of living chain ends be as high as possible at the conclusion of the polymerization reaction. Furthermore, the reaction rate is low and more than 5 or 6 hours of polymerization are often required to end up with commercially relevant monomer conversions.
In short, a method for synthesizing high styrene and high vinyl SSBR with a narrow molecular weight distribution, a monomer conversion higher than 99%, a short polymerization time, and a high amount of living chain ends at the conclusion of the polymerization is highly desirable.